Nanometrology stands for the measurement technology necessary to investigate, to develop and to verify the measuring features of instruments and of materials standards with an uncertainty best given in units of nanometers. For the calibration of different probe systems and different measuring instruments reference standards are asked for.
2-D AFM profilometry uses specially shaped ultrafine silicon tips with a diameter of typically about 250 nm. The absolute measuring preciseness solely depends on the exact knowledge of the tip diameter which usually is determined by a high resolution SEM with a measuring accuracy of 3 to 5%.
With a tip diameter of 250 nm this leads to a measuring preciseness worse than about 7.5 nm. Within the electron microscope, depending on the rest gas concentration, the tip is more or less contaminated and thus its diameter is changed indefinitely during measuring. To improve the measuring accuracy the tip should be calibrated directly before and after the measurement with a gauge.
Other common solutions also focus on the application of single-crystal silicon technology.
A standard reference material (SRM) consisting of a silicon wafer with a silicon dioxide film of uniform thickness is described in SPIE Vol. 661, 402 (1986) "Film thickness and refractive index Standard Reference Material calibrated by ellipsometry and profilometry" by G. A. Candela et.al. The silicon dioxide film contains windows used for stylus profilometry measurements with a mechanical depth very nearly the same as the oxide thickness. The depth is not exactly the same as the oxide thickness due to native oxide forming in the window with an average thickness of about 2 nm. This affects the accuracy of the measurement.
In Metrologia, 1991/92, Vol.28, pp. 443-453, "Nanometrology at the PTB" by H. Kunzmann, a reference scale in the sub-nanometer range is derived from a silicon single crystal epitaxially grown by chemical gas transfer. With high probability the surface of this crystal is plane within one lattice plane and it is proposed to use the steps which are small-integer multiples of lattice plane distances for the calibration of probe systems for nanometrology. These steps are results of the epitaxial processes and they could be used to manufacture step-height gauges, but only if the technology of epitaxial growth could be brought under well-defined metrological control. Although realizing accuracies in the sub-nanometer range, this solution only covers 1-D displacement metrology.